Telecommunications - X.25 Call Setup

1 - Call Setup

1.1 Virtual Circuits

1.2 Successful call set-up

1.3 Unsuccessful call attempt

1.4 Subscription time and call setup time facilities

1.5 - Call user data

The user can include up to 16 bytes of data in the call request packet, 128 bytes for fast select.

This field is passed through the network transparently, The public PAD sets the first 4 bytes, see info on PAD for details.

Protocol identifiers: first 4 bytes of data field

byte number 1 2 3 4


Public Network pad 01 profile speed 00

Telex network adaptor (IS2) 41 00 00 00

Teletex 02 * * *

Network independent transport

Service (yellow book) 7F FF FF FF

Cable TV 60 * * *

*not defined for identification

note profile coded as follows:

PAD Profile Coding
A1 2D
A2 2E
A3 2F
A4 30
A5 31
A6 32
A7 33
A8 34
A9 35
B1 38
B2 39
D1 42
D2 43
SP 58
TP 59
V1 5A
V2 5B
V3 5C
V4 5D
V5 5E

2 - Call Packet





Level 3 facilities

Reverse Charge

Extended Formats

Packet size negotiation

Window size negotiation

Fast select

Closed user group

Call redirection


Call Statistics

International facilities

call is now up and data can be sent

The call request packet is really the same as an incoming call packet, it is known as a call request packet when it goes from the customers equipment to the network and an incoming call packet when it goes from the network to the customers equipment.

Network User Addresses - (NUAs)

9 - Logical Channel Numbers and Logical Channel Group numbers

byte 1 - General Format Identifier and Logical Channel Group Number

D = Delivery Confirmation Bit (D-bit) - see information on data packets

for details, not used on Public Network must

be set to 0.

LCGN = Logical Channel Group Number

this is an extension to the LCN, see below, and it also indicates the type of call (PVC,SVC incoming, bothway or outgoing)

PVC = Permanent Virtual Circuit - not currently supported for new connections to Public Network.

SVC = Switched Virtual Circuit

Byte 2 - Logical Channel Number

This number identifies the call so that subsequent packets on the call do not have to specify the full address/facilities.

When using bothway SVCs it is possible to get a call collision, that is an incoming call being received on a logical channel at exactly the same time as an outgoing call has been sent on that LCN. To minimise this possibility the network always sends incoming calls on the lowest available LCN and the DTE should send on the highest available. If a call collision does occur then the network will proceed with the call request and ignore the incoming call.

8.3 Logical Channel Numbers - LCN

Multiple calls can be sent over a single link, the call is identified by its Logical group Number.

Since it is an 8-bit number it can only identify 256 calls on its own.

8.4 Logical Channel Group Number - LCGN

This is an extension to the LCN (ie a call on LCN=2 LCGN=4 is different from a call on LCN=2 LCGN=5), also the LCGN identifies

the type of call, see table opposite.

Public Network trunks are not strictly X.25 but they use similar principles although they use LCGN=0

PVC = permanent virtual circuit , no longer supported by Public Network for new customers

At subscription time, a customer may request how many incoming, bothway and outgoing channels they wish to subscribe too. Incoming calls, to the customer, start at the lowest LCN,LCGN and work upwards. Outgoing calls, to the network start at the highest LCN,LCGN and work down.

This minimises the chances of a call collision on the same LCN,LCGN and allows the customer to reserve channels for I/C only or O/G only.

The fact that the customer end starts from the highest number means that the customer must know how many channels they have subscribed to when they configure their system.

The LCN,LCGN is independent for each link, so if a call is traced through the network the LCN,LCGN may be different for each link.

Subscription Options

The following subscription options may affect call setup

TOA/NPI address subscription

Incoming Calls Barred

Outgoing Calls Barred

Subscription Option - Direct Call

Extended Packet Sequence Numbering

Local Significance

see D-bit

Subscription Option - D-bit modification



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see also:


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